The invention concerns improvements in guidewires used with balloon dilatation catheters and, particularly, with the over-the-wire type of such catheters used in PTCA, in which a stenosed region of a coronary artery is dilated to increase the blood flow through that artery. The PTCA procedure typically involves advancement of a guide catheter through a percutaneous puncture in the femoral artery to place the distal end of the guide catheter at the entrance (ostium) to one of the two (right or left) main coronary arteries. With the guide catheter properly positioned, a balloon dilatation catheter then is passed through the guide catheter to the ostium of and then into, the coronary arteries. The balloon dilatation catheter typically is used in conjunction with a small diameter steerable guidewire that can be manipulated into the selected arterial branch and through the stenosis that is to be dilated. After the guidewire has been manipulated and navigated into place, the balloon catheter is advanced over and along the guidewire, with the balloon in a deflated state to place the balloon within the stenosis. The balloon then is inflated to dilate the stenosed region of the artery.
Numerous difficulties are presented in the design of a small diameter steerable guidewire for use in PTCA. The difficulties may be appreciated from an understanding of the human arterial anatomy from the usual point of entry, the femoral artery in the groin region, to and including the coronary arteries. The portion of that arterial anatomy is illustrated, in fragmented, somewhat diagrammatic fashion, in FIG. 1. The arterial system carries blood from the heart, indicated as to general region in phantom at 10 through the aortic valve 12 of the heart 10 in a direction indicated by the arrows 13. The arterial system leading from the aortic valve 12 includes, in a downstream direction, the ascending portion 14 of the aorta, the aortic arch, indicated generally at 16 and the remaining (descending) portion 18 of the aorta. Numerous arteries branch off the aorta to carry blood to the internal organs of the body as well as the limbs and extremities, including the iliac arteries 17 and femoral arteries 19 that direct blood to the legs. The coronary artery system (suggested schematically and in part at 20) through which oxygenated blood is directed back to the heart tissue itself includes two main arteries, a left main coronary artery 21 and a right main coronary artery 22, both of which branch off the ascending portion 14 of the aorta close to the aortic valve 12. Each of the left and right coronary arteries 21, 22 leads to a system of numerous branch arteries, some of which are suggested schematically at 20A, 20B, 20C, 20D, that spread out over the wall of the heart muscle thereby serving to distribute oxygenated blood to the entire heart muscle. The object of the PTCA procedure is to treat the portion of an artery that has developed a stenosis, for example, as suggested at 23 which obstructs blood flow through that portion of the artery. The PTCA procedure dilates the stenosis 23 to enlarge the flow area and improve the flow of blood to those portions of the heart tissue served by the stenosed artery.
The PTCA procedure involves initial placement of a comparatively large diameter guide catheter 24 (of the order of about 6 French (0.078" outer diameter) to about 9 French (0.117" outer diameter) through a percutaneous puncture (not shown) in the femoral artery 19. The guide catheter 24 has a specially formed distal end that facilitates engagement of the tip 26 of the guide catheter with the entrance (ostium) 28 to one or the other of the main coronary arteries 21, 22. FIG. 1A illustrates a "Judkins-left" type of guide catheter adapted to intubate the ostium 28 of the left main coronary artery 21. The Judkins-left guide catheter has two principal pre-formed curves at its distal region including a primary curve 30 located approximately one-half to one centimeter from the distal tip 26 of the catheter and a secondary curve 32 located about three to six centimeters from the tip 26. The portion of the guide catheter proximally of the secondary curve 32 is essentially straight all the way to the fitting 25 at the proximal end of the catheter. As illustrated in FIG. 1, when the catheter is deployed in the patient, the secondary curve 32 bears against the wall of the aorta generally opposite the ostium in order to help stabilize the deployed position of the guide catheter. The primary curve serves to direct the tip 26 substantially at the ostium to the left main coronary artery. A normally straight portion of the guide catheter 24 disposed proximally of the secondary curve 32 is sufficiently flexible so that it can be bent through the relatively large radius bend of the region of the aortic arch 16. In this regard, it may be noted that guide catheters typically are formed to include polymeric materials that tend to soften somewhat and become more flexible when exposed to body temperature, thereby facilitating bending of the distal region of the guide catheter to conform to the aortic anatomy in the region of the heart.
As suggested in FIGS. 1 and 1A, the primary and secondary curves 30, 32 of the guide catheter 24, when deployed as well as when relaxed, define a sharper bend than that through which the more proximal portion of the guide catheter assumes when it passes through the aortic arch 16. Typically, the primary and secondary curves 30, 32 may have a radius of the order of one-half to one inch as compared to the radius of the order of one and one-half to two inches for the curve that may be assumed through the aortic arch. The guide catheter 24, once placed, defines a path through and along which an angioplasty catheter (which typically is far more flexible than the guide catheter and about 0.040 inches diameter or less) and guidewire can be advanced easily and quickly to the entrance 28 of the coronary artery. The guide catheter typically is placed in a procedure well-known to those familiar with the art.
In a typical procedure, a small diameter (less than about 0.020 inches and preferably of the order of 0.018 inches or less) steerable guidewire, indicated generally at 34 (FIG. 2) is preloaded into the receptive guidewire lumen (not shown) in the balloon angioplasty catheter, indicated generally at 36. The angioplasty catheter 36 and guidewire 34 are advanced together through the previously placed guide catheter 24 to the ostium 28. Then, while holding the balloon catheter 36 in place within the guide catheter 24, the guidewire is advanced through the balloon catheter into the coronary arteries. The guidewire is manipulated from its proximal end 35 by the physician while the patient is under fluoroscopy so that the distal end of the guidewire can be observed fluoroscopically. The physician, by combined rotational and longitudinal movements of the guidewire 34, can steer the guidewire 34 through the branches of the coronary arterial tree so that the distal end 37 of the guidewire passes through the stenosis 23. Once so positioned the guidewire 34 is held stationary by the physician or an assistant and the balloon catheter 36 then is advanced over and along the guidewire 34, thereby guiding the balloon 40 of the catheter 36 directly to the stenosis 23. With the balloon in place, it then is inflated through an inflation lumen 42, typically with a liquid under high pressure to forcibly dilate the stenosis. It should be understood that for ease of illustration, the stenosis 23 in FIGS. 1 and 2 has been placed in a location in the arterial tree that is relatively free of complex tortuousities and is relatively close to the coronary ostium. It will be appreciated that in order for the guidewire to effectively serve its function of guiding the balloon catheter to the stenosis, the guidewire should be capable of being steered and manipulated into any of the arterial branches such as suggested schematically at 20A-20D as well as other branches located at the most distal portions of the coronary arterial tree. Frequently the stenosis will be located well within a highly tortuous arterial branch of the coronary anatomy such as suggested at 23B in branch artery 20B (FIGS. 1 and 2). In order to reach and treat a stenosis so located, it will be appreciated that the balloon catheter and the guidewire must be steered and advanced through the tortuous anatomy along a path suggested in phantom at 34B in FIG. 2.
In order for the guidewire to perform its function effectively, it should have a number of characteristics. The guidewire should have adequate longitudinal flexibility to enable it to conform to the various curves of the patient's arteries including the frequently highly tortuous configuration of the coronary arteries. It should have adequate column strength so that it can be pushed, as it is advanced through the arteries, without buckling. In order that the guidewire may be steered controllably, it should be sufficiently torsionally rigid to be able to transmit controllably to its distal end substantially all of the rotation applied at the proximal end. The distal tip of the guidewire should be soft and flexible to reduce the risk of injury to the delicate inner lining of the artery. The guidewire also should be kink resistant. Kinking (permanent deformation) in the guidewire typically results in aberrant, uncontrolled whipping movement at the distal tip of the guidewire rather than the desirable controlled transmission of rotation. The guidewire also desirably is highly radiopaque at its distal tip in order that its movement and position may be readily observed under fluoroscopy. Also important among the characteristics of a guidewire is that it have a good tactile response in order that the physician may feel, at the proximal end of the guidewire, events occurring at the distal end.
Since the development of the first small diameter steerable guidewire (Leary U.S. Pat. No. 4,545,390) continued development of small diameter steerable guidewires has involved trade-offs and compromises among the foregoing desirable characteristics. Among these has been the development of guidewires formed from a pseudo-elastic material, such as a nitinol alloy. The pseudo-elastic characteristic of the material provides for excellent kink resistance and a desirably soft, flexible distal tip. Typical of such guidewires are those described in U.S. Pat. No. 4,925,445 (Sakamoto). The advantages of such pseudo-elastic guidewires have been achieved, however, at the expense of other desirable characteristics, particularly in small diameter guidewires of the type now commonly used in PTCA, of the order of 0.014 inches to as small as 0.010 inches diameter. Although the performance of pseudo-elastic guidewires may be less problematic in larger sizes, the performance may become marginal when the diameter is as small as 0.014" and poor in smaller sizes. Performance becomes marginal to poor, particularly with respect to the column strength of the guidewire and its ability to be pushed without buckling. Similarly, pseudo-elastic guidewires of the order of 0.014 inch diameter and smaller, adapted for use in PTCA tend to display marginal to poor steerability characteristics. These disadvantages also compromise the tactile response of the wire.
It would be desirable to provide a small diameter steerable guidewire in which the foregoing desirable characteristics are maximized, with a minimum amount of compromise of one characteristic for another. It is a general object of the invention to provide such a guidewire.